Four-wheel drive system with center differential lock control responsive to rpm difference

ABSTRACT

A four-wheel drive system of a vehicle has a center differential between front wheels and rear wheels, and means for restraining or locking the center differential. The four-wheel drive system is combined with a control system for automatically locking the center differential when the difference between an average rotational speed of the right and left front wheels and an average rotational speed of the right and left rear wheels becomes equal to or larger than a predetermined value.

BACKGROUND OF THE INVENTION

The present invention relates to a four-wheel drive motor vehicle of atype having a center differential between the front and rear wheels, andmeans for restraining or locking the center differential. Morespecifically, the present invention relates to a control system forlocking a center differential of a four-wheel drive system in accordancewith a rotational speed difference between the front wheels and the rearwheels.

A four-wheel drive vehicle is superior in ability of hill climbing andability of rough road driving. If, however, all four wheels are alwaysdriven at the same speed, a four-wheel drive vehicle cannot be turnedsmoothly. During a turn with a large steering angle of steerable wheels,the front wheel of an inner or outer side must travel along a circlehaving a large radius and the rear wheel of the same side must travelalong a small circle. Because of this difference in turning radius,there arises a large difference between a rotation speed (an averagerotation speed, to be exact) of the front wheels and a rotation speed(an average rotation speed) of the rear wheels. As a result, thesteering becomes heavy, the tendency to understeer is increased, and thevehicle cannot be turned without abnormal tire friction (called tightcorner brake) which tends to brake the vehicle and cause an enginestall.

To overcome these handling and tire friction problems, some four-wheeldrive systems use a center

differential between front and rear wheels. If, however, one of the fourwheels run on a muddy place, the center differential coacts with a rearor front differential between right and left wheels, so that the torquecannot be transmitted to the other three wheels, and the vehicle cannotescape from the muddy place.

In view of this problem, some four-wheel drive systems are furtherprovided with means (a lockup mechanism or a non-slip differentialmechanism) for locking the center differential. One example is shown inJapanese patent provisional publication No. 57-114727.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve a four-wheel drivevehicle of a type having a center differential and means for locking orrestraining the center differential.

According to the present invention, a four-wheel drive vehicle comprisesan engine, front right and left wheels, rear right and left wheels, afour-wheel drive system and a control system. The four-wheel drivesystem is capable of transmitting power from the engine to the frontwheels and the rear wheels. The four-wheel drive system comprises acenter differential connected between the front wheels and the rearwheels for absorbing a rotational speed difference between the frontwheels and the rear wheels, and restraining means capable of holding thecenter differential in a restrained condition in which the differentialaction of the center differential is restrained, and in a releasedcondition in which the differential action of the center differential isallowed. The four-wheel drive system further comprises front wheel siderotating means connected between the center differential and the frontwheels, and rear wheel side rotating means connected between the centerdifferential and the rear wheels. The center differential allows thefront wheel side rotating means and the rear wheels side rotating meansto rotate at different speeds when the center differential is in thereleased condition. The control system comprises front speed sensingmeans for sensing a rotational speed of the front wheel side rotatingmeans, rear speed sensing means for sensing a rotational speed of therear wheel side rotating means, condition sensing means for detectingwhether the center differential is in or out of the restrainedcondition, and control means connected with both of the front and rearspeed sensing means and the condition sensing means. The control meanscontrols the restraining means in accordance with signals of the frontand rear speed sensing means and the condition sensing means. Thecontrol means commands the restraining means to bring the centerdifferential to the restrained condition if the difference between therotational speed of the front wheel side rotating means and therotational speed of the rear wheel side rotating means is equal to orlarger than a predetermined value and at the same time the centerdifferential is out of the restrained condition.

In one embodiment, the front wheel side rotating means comprises a frontright axle shaft connected with the front right wheel and a front leftaxle shaft connected with the front left wheel, and the rear wheel siderotating means comprises a rear right axle shaft connected with the rearright wheel and a rear left axle shaft connected with the rear leftwheel. The front speed sensing means senses rotational speeds of thefront right wheel and the front left wheel, and the rear speed sensingmeans senses rotational speeds of the rear right wheel and the rear leftwheel. The control means commands the restraining means to bring thecenter differential to the restrained condition if the differencebetween an average rotational speed of the front wheels and an averagerotational speed of the rear wheels is equal to or larger than apredetermined value and at the same time the center differential is outof the restrained condition.

In another embodiment, the front wheel side rotating means is adifferential case of a front differential connected between the frontwheels for absorbing a rotational speed difference between the frontwheels, and the rear wheel side rotating means is a differential case ofa rear differential connected between the rear wheels for absorbing arotational speed difference between the rear wheels.

In still another embodiment, the front wheel side rotating means and therear wheel side rotating means are side gears of the centerdifferential.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a vehicle, for showing a turning radiusdifference between front wheels and rear wheels when a steering angle ofsteerable wheels are large,

FIG. 2 is a schematic view of a four-wheel drive vehicle of a firstembodiment of the present invention,

FIG. 3 is a fragmentary sectional schematic view of a centerdifferential restraining means used in the vehicle of FIG. 2,

FIG. 4A is a block diagram of a control system of the first embodiment,

FIG. 4(B) is a flowchart for the control system of the FIG. 4(A).

FIG. 5 is a schematic view of a four-wheel drive vehicle of a secondembodiment of the present invention,

FIG. 6 is a block diagram of a control system of the second embodiment,

FIG. 7 is a schematic view of a four-wheel drive vehicle of a thirdembodiment of the present invention, and

FIG. 8 is a block diagram of a control system of the third embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the present invention is shown in FIG. 2. Anengine 2 is supported on a vehicle body 1. The engines is connected to atransmission 3. The transmission 3 is connected to a transfer ortransfer case 4. The transfer 4 has a high-low two-speed change unit 5.The transmission 3 is connected to the high-low change unit 5 of thetransfer 4. The high-low change unit 5 has an output shaft 5a. Thetransfer 4 further has a center differential unit 7. The output shaft 5aof the high-low unit 5 is connected to the center differential unit 7.The center differential unit 7 has a differential case 7a, a pinionshaft 7b fixed to the differential case 7a, two differential pinions 7crotatably mounted on the pinion shaft 7b, and front and rear side gears7d which mesh with the differential pinions 7c.

The rear side gears 7d is connected to one end of a rear propeller shaft13. The other end of the rear propeller shaft 13 is connected with arear differential unit 14. The rear differential unit 14 is connected torear wheels 16a and 16b through rear axle shafts 15a and 15b. The reardifferential unit 14 allows the right and left rear wheels 16a and 16bto rotate at different speeds, and thereby absorbs a rotational speeddifference between the right and left rear wheels 16a and 16b.

There are provided rear wheel rpm sensors 17a and 17b mounted on thevehicle body 1 near the rear axle shafts 15a and 15b, respectively. Therear wheel rpm sensors 17a and 17b, respectively, sense rotationalspeeds of the right and left rear wheels 16a and 16b in revolutions perunit time.

The front side gear 7d of the center differential unit 7 is connectedwith a first chain wheel 18 placed coaxially with the front side gear7d. The first chain wheel 18 is connected with a second chain wheel 20by a chain belt 19. A shaft of the second chain wheel 20 is connectedwith one end of a front propeller shaft 21. The other end of the frontpropeller shaft 21 is connected to a front differential unit 22. Thefront differential unit 22 is connected with front wheels 25a and 25bthrough front axle shafts 24a and 24b, respectively. The frontdifferential unit 22 allows the right and left front wheels 25a and 25bto rotate at different speeds, and thereby absorbs a rotational speeddifference between the right and left front wheels 25a and 25b.

There are provided front wheel rpm sensors 12a and 12b mounted on thevehicle body near the front axle shafts 24a and 24b, respectively. Thefront wheel rpm sensors 12a and 12b, respectively, sense rotationalspeeds of the right and left front wheels 25a and 25b in revolutions perunit time.

The transfer 4 further has a center differential restrain unit 26disposed between the differential case 7a of the center differentialunit 7 and the first chain wheel 18. As shown in FIG. 3, the centerdifferential restrain unit 26 has a center differential control valve 27connected with a hydraulic system of the vehicle such as the hydraulicsystem of the transmission 3. The center differential control valve 27has a spool 27a and a spring 27b. The center differential restrain unit26 further has a solenoid 23 having an iron core 23a for moving thespool 27a against the force of the spring 27b. The solenoid 23 serves asan actuator of a control system. The center differential restrain unit26 further has a hydraulic type multiple disc clutch 29. The clutch 29is operated by an oil pressure of an oil chamber 29a. The oil chamber29a is fluidly connected with a port of the center differential controlvalve 27. The multiple disc clutch 29 has a group of first clutch plates29b and a group of second clutch plates 29c. One of the two groups isconnected with the differential case 7a of the center differential unit7. The other is connected with the first chain wheel 18. The centerdifferential control valve 27 can move the spool 27a with the iron core23a in accordance with the current passing through the solenoid 23, andthereby control the fluid pressure in the oil chamber 29a by controllingthe fluid flow. The clutch 29 is frictionally engaged when the oilpressure in the oil chamber 29a is increased by moving the spool 27adownwards against the spring 27b. The clutch 29 is in a slipping statewhen the oil pressure in the oil chamber 29a is decreased by moving thespool 27a upwards in FIG. 3 with the aid of the spring 27b. The clutch29 is disengaged when the oil is drained from the oil chamber 29a.

There is further provided a sensor 31 for detecting the condition of thecenter differential 7. In this embodiment, the differential conditionsensor 31 detects whether the clutch 29 is engaged or not by monitoringthe oil pressure in the oil chamber 29a.

A control unit or circuit 32 is connected with the front wheel rpmsensors 12a and 12b, the rear wheel rpm sensors 17a and 17b, and thedifferential condition sensor 31 for receiving signals from thesesensors. The control circuit 32 produces a control signal in accordancewith the signals of these sensors, and sends the control signal to thesolenoid (actuator) 23. Thus, a control system is formed as shown inFIG. 4(A).

When the center differential restrain unit 26 is not actuated, thecenter differential unit 7 works in the following manner. When thesteering angle θ of the steerable front wheels 25a and 25b is increased,the radii (R) of arc-shaped paths traveled by the front wheels 25a and25b become largely different from the radii (r) of arc-shaped pathstraveled by the rear wheels 16a and 16b, as shown in FIG. 1. In FIG. 1,only the paths of the inside front and rear wheels are shown.Consequently, there arises a large difference between an averagerotation speed of the front wheels 25a and 25b and an average rotationspeed of the rear-wheels 16a and 16b. The center differential 7 in itsreleased state absorbs this average rotation speed difference betweenthe front and rear wheels, so that the vehicle can turn smoothly thoughthe steering angle θ is large.

If the vehicle travels on an off road with the center differential 7being held in its released position, and any one of the four wheels(25a, for example) sinks in a mire and begins slipping, then all theother three wheels 25b, 16a and 16b lose their power for driving thevehicle because of the cooperation of the center differential 7 and thefront differential 22 or the rear differential 14. As a result, thevehilcle cannot escape from the mire. In this case, the wheel 25b pairedwith the slipping wheel 25a can hardly rotate because of roadresistance, so that there arises a large difference of rotational speedbetween the right and left wheels 25a and 25b. This rotational speeddifference is absorbed by the front differential 22 disposed between thewheels 25a and 25b. The rear wheels 16a and 16b can hardly rotatebecause of road resistance. Almost all the rotation of the output shaft5a of the high-low unit 5 is wasted on rotating the slipping wheel 25 a.The front propeller shaft 21 rotates fast but the rear propeller shaft13 remains almost stationary. There arises a large rotational speeddifference between the front wheel side and the rear wheel side of thecenter differential 7.

This rotational speed difference is monitored by the control system ofthe present invention. In the first embodiment, the front wheel rpmsensors 12a and 12b sense, respectively, the rotational speed of thefront right axle shaft 24a, i.e. the rotational speed (Nfr) of the frontright wheel 25a, and the rotational speed of the front left axle shaft24b, i.e. the rotational speed (Nfl) of the front left wheel 25b. Therear wheel rpm sensors 17a and 17b sense, respectively, the rotationalspeed of the rear right axle shaft 15a, i.e. the rotational speed (Nrr)of the rear right wheel 16a, and the rotational speed of the rear leftaxle shaft 15b, i.e. the rotational speed (Nrl) of the rear left wheel16b. The control unit 32 receives signals indicative of the sensedspeeds from the front and rear wheel rpm sensors 12a, 12b, 17a and 17b.From these signals, the control unit 32 calculates an average frontwheel rotational speed N_(F) given by N_(F) =(Nfr+Nfl)/2, and an averagerear wheel rotational speed N_(R) given by N_(R) =(Nrr+Nrl)/2, as shownin FIG. 4B. Then, the control unit 32 finds the absolute value of thedifference between the average front wheel rotational speed N_(F) andthe average rear wheel rotational speed N_(R). If this absolute value ofthe rotational speed difference becomes equal to or greater than apredetermined value (M), and at the same time the center differential 7is held in the released condition, then the control unit 32 commands thecenter differential restrain unit 26 to engage the clutch 29 by sendingthe control signal to the solenoid 23. In response to the controlsignal, the current supply to the solenoid 23 is gradually increased,and accordingly the center differential control valve 27 increases theoil pressure in the oil chamber 29a by moving the spool 27a downwards inFIG. 3. When the oil pressure in the oil chamber 29a becomes high, theclutch 29 is engaged, and the center differential 7 is locked so thatits function is prevented. By locking the center differential 7, thecenter differential restrain unit 26 prevents a relative rotationbetween the front propeller shaft 21 and the rear propeller shaft 13.Therefore, the driving torque can be transmitted to the rear wheels 16aand 16b which are not stuck in the mire, and the vehicle can easilyescape from the mire.

A second embodiment of the present invention is shown in FIGS. 5 and 6.In the second embodiment, the front wheel rpm sensors 12a and 12b andthe rear wheel rpm sensors 17a and 17b are replaced by a frontdifferential case rpm sensor 22a for sensing a rotational speed of adifferential case of the front differential 22, and a rear differentialcase rpm sensor 14a for sensing a rotational speed of a differentialcase of the rear differential 14. Each of the front and reardifferentials 22 and 14 has a differential case adapted to be driven bythe engine 2 through the center differential 7, a pinion shaft fixed tothe differential case, two differential pinion rotatably mounted on thepinion shaft and right and left side gears engaging with the pinions. Ineach of the front and rear differentials 22 and 14, the differentialcase rotates at a speed equal to one half the sum of the right and leftwheel speeds, and the differential case is connected with the propellershaft 21 or 13 through gears having a constant gear reduction ratio.Therefore, the control system of the second embodiment can determines arotational speed difference between the front wheels and the rearwheels. As shown in FIG. 6, the control unit 32 is connected with therear differential case rpm sensor 14a, the front differential casesensor 22a and the condition sensor 31. If the absolute value of thedifference between the front differential case rotating speed and therear differential rotating speed becomes equal to or greater than apredetermined value, and at the same time the center differential 7 isnot locked, then the control unit 32 sends the control signal to thesolenoid 23 and commands the center differential restrain unit 26 toengage the clutch 29 and to lock the center differential 7. In thesecond embodiment, the number of the required sensors is small so thatthe circuitry of the control unit 23 can be simplified as compared withthe first embodiment.

A third embodiment of the present invention is shown in FIGS. 7 and 8.In the third embodiment, there are provided a front side gear rpm sensor34 for sensing a rotational speed of the front side gear 7d of thecenter differential 7, and a rear side gear rpm sensor 35 for sensing arotational speed of the rear side gear 7d of the center differential 7,in place of the wheel rpm sensors of the first embodiment or thedifferential case rpm sensors of the second embodiment. The front sidegear rpm sensor 34 is disposed near the shaft of the front side gear 7dwhich is connected with the front propeller shaft 21. The rear side gearrpm sensor 35 is disposed near the shaft of the rear side gear 7d whichis connected with the rear propeller shaft 13. The control unit 32 ofthe third embodiment is connected with the front side gear rpm sensor34, the rear side gear rpm sensor 35 and the condition sensor 31, asshown in FIG. 8. If the absolute value of the difference betweenrotational speeds of the front and rear side gears of the centerdifferential 7 becomes equal to or greater than a predetermined value,and at the same time the center differential 7 is not locked, then thecontrol unit 32 sends the control signal to the solenoid 23 and commandsthe center differential restrain unit 26 to engage the clutch 29 and tolock the center differential 7. In the third embodiment, the number ofthe required sensors is small, and the circuitry of the control unit 23can be simplified as in the second embodiment.

Thus, the control system of the present invention automatically locksthe center differential when at least one of the wheels begins slipping,and thereby makes the four-wheel drive vehicle very easy to control evenfor unskilled drivers.

What is claimed is:
 1. A four-wheel drive vehicle comprising:an engine,a front right wheel, a front left wheel, a rear right wheel and a rearleft wheel, a four-wheel drive system for transmitting power from saidengine to said front wheels and said rear wheels, said four-wheel drivesystem comprising a center differential connected between said frontwheels and said rear wheels for absorbing a rotational speed differencebetween said front wheels and said rear wheels, and restraining meanscapable of holding said center differential selectively in a restrainedcondition in which the differential action of said center differentialis restrained, and in a released condition in which the differentialaction of said center differential is allowed, said four-wheel drivesystem further comprising front wheel side rotating means connectedbetween said center differential and said front wheels for driving saidfront wheels and rear wheel side rotating means connected between saidcenter differential and said rear wheels for driving said rear wheels,said center differential allowing said front wheel side rotating meansand said rear wheel side rotating means to rotate at different speedswhen said center differential is in the released condition, and acontrol system comprising:front speed sensing means for sensing arotational speed of said front wheel side rotating means, rear speedsensing means for sensing a rotational speed of said rear wheel siderotating means, condition sensing means for detecting whether saidcenter differential is in or out of said restrained condition, andcontrol means, connected with both of said front and rear speed sensingmeans and said condition sensing means, for controlling said restrainingmeans in accordance with signals of said front and rear speed sensingmeans and said condition sensing means, said control means commandingsaid restraining means to bring said center differential to therestrained condition if the difference between the rotational speed ofsaid front wheel side rotating means and the rotational speed of saidrear wheel side rotating means is equal to or larger than apredetermined value and at the same time said center differential is outof the restrained condition.
 2. A vehicle according to claim 1, whereinsaid center differential comprises center input means adapted to bedriven by said engine, and a pair of center front output means andcenter rear output means which are both driven by said center inputmeans, said center front output means being connected with said frontwheels for driving said front wheels and said center rear output meansbeing connected with said rear wheels for driving said rear wheels, saidcenter differential allowing said center front output means and saidcenter rear output means to rotate at different speeds when said centerdifferential is in the released condition.
 3. A vehicle according toclaim 2, wherein a relative rotation between said center front outputmeans and said center rear output means is prevented by said restrainingmeans when said center differential is in the restrained condition.
 4. Avehicle according to claim 3, wherein said front wheel side rotatingmeans comprises front right rotating means which is connected with saidfront right wheel so that said front right wheel and rotating meansrotate together, and front left rotating means which is connected withsaid front left wheel so that said front left wheel and rotating meansrotate together, and said rear wheel side rotating means comprises rearright rotating means which is connected with rear right wheel so thatsaid rear right wheel and rotating means rotate together, and rear leftrotating means which is connected with said rear left wheel so that saidrear left wheel and rotating means rotate together, wherein said frontspeed sensing means comprises front right speed sensing means forsensing a rotational speed of said front right rotating means and frontleft speed sensing means for sensing a rotational speed of said frontleft rotating means, and said rear speed sensing means comprises rearright speed sensing means for sensing a rotational speed of said rearright rotating means and rear left speed sensing means for sensing arotational speed of said rear left rotating means, and wherein saidcontrol means regards the rotational speed of said front wheel siderotating means as being equal to an average rotational speed of saidfront right rotating means and said front left rotating means, andregards the rotational speed of said rear wheel side rotating means asbeing equal to an average rotational speed of said rear right rotatingmeans and said rear left rotating means.
 5. A vehicle according to claim4, wherein said front right, front left, rear right and rear leftrotating means are axle shafts.
 6. A vehicle according to claim 5,wherein said four-wheel drive system further comprises a frontdifferential connected between said front right wheel and said frontleft wheel for allowing both to rotate at different speeds, and a reardifferential connected between said rear right wheel and said rear leftwheel for allowing both to rotate at different speeds.
 7. A vehicleaccording to claim 6, wherein said center input means of said centerdifferential comprises a center differential case and at least twopinions mounted on said center differential case, said center frontoutput means being a center front side gear and said center rear outputmeans is a center rear side gear, said center front and center rear sidegears being in mesh with said pinions.
 8. A vehicle according to claim7, wherein said restraining means comprises a friction clutch disposedbetween said center differential case and said center front side gear,said clutch holding said center differetial in the restrained conditionwhen said clutcn is engaged, and in the released condition when saidclutch is disengaged.
 9. A vehicle according to claim 8, wherein saidcondition sensing means detects the condition of said centerdifferential by detecting whether said clutch is engaged or disengaged.10. A vehicle according to claim 9, wherein said clutch is operated by afluid pressure, and said restraining means further comprises a solenoidvalve for controlling the fluid pressure.
 11. A vehicle according toclaim 10, wherein said clutch holding said center differential in acondition intermediate between the restrained condition and the releasedcondition when said clutch is slippingly engaged.
 12. A vehicleaccording to claim 3, said drive system further comprises a frontdifferential which comprises front input means connected with saidcenter front output means, and a pair of front right output means andfront left output means which are both driven by said center frontoutput means through said front input means, said front right outputmeans being connected with said front right wheel and said front leftoutput means being connected with said front left wheel, said frontdifferential being capable of allowing said front right output means andsaid front left output means to rotate at different speeds, and saiddrive system further comprises a rear differential which comprises rearinput means connected with said center rear output means and a pair ofrear right output means and rear left output means which are both drivenby said center rear output means through said rear input means, saidrear right output means being connected with said rear right wheel andsaid rear left output means being connected with said rear left wheel,said rear differential being capable of allowing said rear right outputmeans and said rear left output means to rotate at different speeds, andwherein said front side rotating means is said front input means of saidfront differential, and said rear wheel side rotating means is said rearinput means of said rear differential.
 13. A vehicle according to claim12, wherein said front input means of said front differential comprisesa front differential case and at least two pinions mounted on said frontdifferential case, said front right and front left output means beingside gears which are in mesh with said pinions of said frontdifferential, said rear input means of said rear differential comprisinga rear differential case and at least two pinions mounted on said reardifferential case, said rear right and rear left output means being sidegears which are in mesh with said pinions of said rear differential,said front speed sensing means sensing a rotational speed of said frontdifferential case and said rear speed sensing means sensing a rotationalspeed of said rear differential case.
 14. A vehicle according to claim3, wherein said front wheel side rotating means is said center frontoutput means of said center differential, and said rear wheel siderotating means is said center rear output means of said centerdifferential.
 15. A vehicle according to claim 14, wherein said centerinput means of said center differential comprises a center differentialcase and at least two pinions mounted on said center differential case,said center front output means and said center rear output means beingside gears meshing with said pinions.
 16. A vehicle according to claim15, wherein said drive system further comprises a front differentialwhich comprises front input means connected with said center frontoutput means, and a pair of front right output means and front leftoutput means which are both driven by said center front output meansthrough said front input means, said front right output means beingconnected with said front right wheel and said front left output meansbeing connected with said front left wheel, said front differentialbeing capable of allowing said front right output means and said frontleft out means to rotate at different speeds, and said drive systemfurther comprises a rear differential which comprises rear input meansconnected with said center rear output means and a pair of rear rightoutput means and rear left output means which are both driven by saidcenter rear output means through said rear input means, said rear rightoutput means being connected with said rear right wheel and said rearleft output means being connected with said rear left wheel, said reardifferential being capable of allowing said rear right output means andsaid rear left output means to rotate at different speeds.